Bound states in the continuum through quantum interference in ultracold atomic collisions

Abstract Quantum-mechanical interaction potentials support localized bound states below the dissociation threshold or delocalized scattering states above it. Although bound states are energetically isolated, scattering states couple a quantum system to environmental modes, making dissipation and loss intrinsic to open systems. Bound states in the continuum are an exception, as destructive interference keeps them localized within the scattering continuum. Theory predicts that such states can arise through interference between two Feshbach resonances coupled to a common continuum. Here we form such an interference-stabilized state in ultracold collisions of 6 Li atoms by coherently coupling two tunable Feshbach resonances using Floquet engineering. At a critical parameter point, elastic and inelastic coupling to the continuum vanish, producing a molecular state above the dissociation threshold. Consequently, resonant scattering features disappear and the colliding atoms become effectively transparent to one another. Full coupled-channel calculations and a minimal non-Hermitian model quantitatively reproduce our observations and identify the state as a Friedrich–Wintgen bound state in the continuum. These results show how quantum interference can control openness in quantum matter and enable the engineering of non-Hermitian Hamiltonians.